Leaves collectively constitute the greatest manufacturing plant in the world. Most of the food, clothing, and medicine used by man is formed as a result of the work of the leaf. The cell contents, structure, and arrangement of the different cells of the leaf differ in a marked degree from the cell contents, structure, and arrangement of the cells in the other organs of the plant. This accounts for the presence of the large amount of chlorophyll in the leaf, the presence of stomata, and the peculiar arrangement of the cells.

It should be ascertained if the stomata are above, even with, or below the epidermis; the nature of the epidermal cells, and, when present, the nature of the hypodermal.cells; the number of layers of palisade parenchyma and whether it is present on both surfaces of the leaf, and the nature of the outgrowths from the epidermal cells.

Klip Buchu

The cross-section of klip buchu (Plate 108) has the following structure:

Epidermis

The epidermal cells of klip buchu are modified to form papillae, the walls are yellowish white, and the papillate portion of the cell is nearly solid.

Hypodermis

The hypodermal cells are never intact because the mucilage contained in the cells swells when placed in water and breaks the thin side walls.

Upper Palisade Parenchyma

The palisade parenchyma is two layers in thickness. The cells of the outer layer are greatly elongated and are packed with chlorophyll. The inner layer of palisade cells is more irregular, and the cells are much shorter than the cells of the outer palisade layer.

Spongy Parenchyma

The spongy parenchyma cells are branched; therefore, large intercellular spaces occur between the cells.

Under Palisade Parenchyma

The palisade cells of the under epidermis are short and broad, and they contain fewer chlorophyll grains than the upper palisade cells of the upper epidermis. These cells occur only under the veins.

Under Hypodermis

The under hypodermal cells are shorter and broader than the upper hypodermal cells.

Under Epidermis

The under epidermal cells are modified to form papillae which are similar to the papillae of the upper epidermis.

Fibro-Vascular Bundle

The cells composing the vascular bundle are sieve cells, vessels, and fibres.

The sieve cells are small and the walls are white and angled.

The vessels have thick, white, angled walls.

The bast fibres are rounded in outline and the walls are thick and white.

Endodermis

The endodermal cells encircle the fibro-vascu-lar bundles. The cells are large, thin-walled, and oval in shape.

Secretion Cells

Near the edges of the leaf are schizoge-nous secretion cavities surrounded by thin-walled secretion cells.

Plate 108. Cross-Section of Klip Buchu just over the Vein.

A. Papillate upper epidermis.

B. Hypodermal cells with broken side walls, due to expansion of mucilage contents.

C. Palisade cells, showing two cells filled with chlorophyll.

D. Palisade like mesophyll.

E. Endodermis.

F. Vascular strand of vein.

G. Conducting cells with spirally thickened walls. H. Characteristic leaf mesophyll.

I. Short, thick palisade cells on the under side of leaf, just under the vein. J. Under hypodermal cells. K. Papillate under epidermis.

Powdered Klip Buchu

When the leaf is powdered (Plate 109), the cells are quite as characteristic in appearance. The upper epidermal cells (1) have thick-beaded, yellowish-white walls and papillate outer walls. No stomata occur on the upper surface. The under epidermis (2) with numerous stomata, is surrounded by the characteristic guard cells. The end walls are beaded as on the upper surface. The palisade cells (3) appear as in the cross-section. The conducting cells (4 and 4) are of the spiral and pitted type. The papillae (5 and 5) are very abundant in the powder and very characteristic. The fragments of the epidermis (6) are also abundant. The mesophyll (7) is characteristic, as it retains its form when powdered. The fibres (8) are usually associated with the conducting cells; occasionally they are found free as in the illustration.

Plate 109. Powdered Klip Buchu.

I. Upper epidermis. 2. Under epidermis. 3. Palisade cells with chlorophyll. 4 and 4. Conducting cells. 5 and 5. Papillae. 6. Fragments of the epidermis. 7. Mesophyll. 8. Fibres.

Mountain Laurel

Epidermis

The epidermal cells of mountain laurel are occasionally modified, as unicellular hairs (Plate no, Fig. 1), particularly in the region of the veins. The ordinary epidermal cells have thick outer walls and thin inner walls. Beneath many of the epidermal cells are large air-spaces.

Upper Palisade Parenchyma

The palisade parenchyma vary from four to five layers. The inner palisade cells are shorter and broader than the outer layer of cells.

Parenchyma

The parenchyma cells (Fig. 4) are rounded in form and they are arranged in the form of columns which are one cell in thickness above, but two to three cells in thickness near the under epidermis. Between each chain of cells is a larger intercellular space (Fig. 6). In a few of the cells are large rosette crystals.